This invention relates to an improvement in rubber sealing devices, for example, oil seals, rolling bearing seals, and rubber seals in linear acting apparatuses such as linear guides and ball screws which are employed to prevent the leakage of greases or lubricants and the invasion of foreign materials from the outside.
Linear acting apparatuses such as linear guides and ball screws, oil seals and rolling bearings are provided with rubber sealing devices in order to prevent the leakage of greases or lubricants and the invasion of foreign materials from the outside.
These rubber sealing devices usually have a unified structure wherein a reinforcing member made of a metal is joined to a rubber composition with the use of an adhesive. As the rubber composition, acrylonitrile butadiene rubber (NBR) is most commonly employed by taking oil resistance into consideration. In case where particularly high heat resistance and chemical resistance are required, use is made of acrylic rubbers, silicone rubbers and fluororubbers (for example, vinylidene fluoride-hexafluoropropylene and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene commonly called FKM).
In recent years, spindle speeds of machine tools, in which many linear acting apparatuses, oil seals and rolling bearings are employed, have been elevated to satisfy the requirement for improving the processing performance. For example, in a linear guide frequently employed in cutting tools, the temperature at the cutting point is elevated with an increase in the spindle speed. As a result, it is required that a cutting oil solution has such a high permeability as to instantaneously attain the cutting point as well as an improved cooling performance. It is also required that a cutting oil solution sustains its lubricating properties at the cutting point at a high temperature under high pressure.
To satisfy these requirements, there have been developed and spread soluble-type synthetic cutting coolants, which are completely soluble in water, as substitutes for the conventional emulsion-type water-soluble cutting oils. These soluble-type synthetic cutting coolants are advantageous from the environmental viewpoint too, since the waste water can be easily processed. However, these soluble-type synthetic cutting coolants contain a large amount of various additives such as amines to elevate the permeability and, therefore, show a high permeability into rubbers constituting rubber sealing devices too, thereby frequently causing deteriorations (swelling, softening, etc.) of rubbers. As a result, the rubber strength at the lips of rubber sealing devices is weakened. In case of a lip of the contact type, in particular, the exposed thread of the seal is enlarged and thus the abrasion is accelerated. Thus, the sealing properties are worsened. Moreover, the life of the member provided with the seal per se is shortened thereby. In a fluororubber (FKM), in particular, the vinylidene fluoride moiety in the structure easily undergoes de-HF due to the amine-based additives and thus the deterioration of the rubber is accelerated. Consequently, the rubber strength is further lowered in many cases.
These amine-based additives sometimes cause deterioration of adhesives by which reinforcing members are joined to rubbers. In a nitrile rubber sealing device, for example, a reinforcing member is joined to the nitrile rubber usually with a phenol-based adhesive. When this sealing device is immersed over a long time in a soluble-type synthetic cutting coolant diluted with water, it is sometimes observed that the nitrile rubber is swollen and softened and the phenol-based adhesive is deteriorated so that the nitrile rubber completely peels off from the reinforcing member.
Since automotive engine oils and antifreezes also contain amine-based additives in many cases, it is feared that the same troubles might arise in automotive rolling bearings.
The invention, which has been completed under these circumstances, aims at providing long-life rubber sealing devices which are highly tolerant to liquids containing amine-based additives (in particular, soluble-type synthetic cutting coolants suitable for high-speed processing, automotive coolants and engine oils) and can sustain favorable sealing performance over a long period of time and rubber compositions appropriate therefor.
To attain this aim, the invention provides a rubber composition to be joined to a reinforcing member of a rubber sealing device which contains a tetrafluoroethylene-propylene bipolymer and from 0.02 to 10 parts by weight of a liquid polybutadiene per 100 parts by weight of the tetrafluoroethylene-propylene bipolymer.
The invention further provides a rubber sealing device having a reinforcing member and a rubber composition joined thereto, wherein the rubber composition contains a tetrafluoroethylene-propylene bipolymer and from 0.02 to 10 parts by weight of a liquid polybutadiene per 100 parts by weight of the tetrafluoroethylene-propylene bipolymer.